The nature of the cell membrane receptor to which glucose and other monosaccharides are bound prior to and during their penetration into the cell is being studied. Phlorizin and a number of organically synthesized compounds related in structure to phlorizin have been found to be preferentially bound to this glucose transport receptor and thus inhibit the membrane penetration of sugars. This characteristic binding property serves as a fingerprint which can be used to identify the membrane component in attempts to isolate it from the cell membranes of a number of mammalian tissues. Radioactively labelled phlorizin and phlorizin analogs have been prepared to serve as easily measurable marker molecules in these investigations.